THE TRUTH ABOUT MAGNETIC FLUX LEAKAGE AS APPLIED TO TANK FLOOR INSPECTIONS
Magnetic Flux Leakage (MFL) Inspection techniques have been widely used in the NDT oil field inspection industry for over a quarter of a century for the examination of pipe, tubing and casing, both new and used. It is only in the last fifteen years that MFL inspection techniques have been applied to above ground storage tank floors in an attempt to provide a reliable indication of the overall floor condition within an economical time frame. In most cases, these MFL inspections are being carried out by industrial inspection NDT companies who do not have the depth of experience in the technique that most of the oil field tubular inspection companies have.
At the same time this relatively new application of Magnetic Flux Leakage brings with it some additional problems not evident in the inspection of tubulars where certain parameters can be quite closely controlled. Probably the greatest of these is that tank floors are never flat, whereas tubulars are generally always round. The ability to obtain any reasonably consistent quantitative information is seriously impacted by this general unevenness of most tank floors. The application of rigid accept/reject criteria based on signal amplitude thresholds has proved to be absolutely unreliable as regards truly quantitative information. A more realistic approach is required in the application of this NDT inspection technique and in the design of the MFL inspection equipment to ensure that there are fewer incidences of significant defects being missed.
The following information outlines some of the major considerations that need to be addressed in order to achieve reliable, fast and economical inspections of above ground storage tank floors.
Magnetic Flux Leakage (MFL)
In order to understand some of the problems associated with this particular application of Magnetic Flux Leakage (MFL), it is necessary to understand the basic principles of the technique. Most people are familiar with a magnet’s ability to “stick” to a carbon steel plate. This happens because the magnetic lines of force (flux) prefer to travel in the carbon steel plate rather than in the surrounding air. In fact, this flux is very reluctant to travel in air unless it is forced to do so by the lack of another suitable medium. For the purposes of this particular application, a magnetic bridge is used to introduce as near a saturation of flux as is possible in the inspection material between the poles of the bridge. Any significant reduction in the thickness of the plate will result in some of the magnetic flux being forced into the air around the area of reduction. Sensors which can detect these flux leakages are placed between the poles of the bridge. Figure 1 graphically illustrates this phenomenon.
The MFL Inspection Environment
In order to optimize the effectiveness of the MFL inspection, it is necessary to consider the environment and address the physical restrictions imposed by the actual conditions found when examining the majority of tank floors.
Invariably, the range of temperature and humidity conditions will vary enormously worldwide. The effect on both operator and equipment must be taken into consideration. Human beings do not function well in extremes of temperature. Use of the MFL inspection equipment should not place too great a burden on them from either a physical or mental point of view. In other words, the simpler, more reliable and easy to use the MFL inspection equipment is made, the more reliable the inspection results.
Tank Floor Cleanliness
By their very nature, the majority of above ground storage tanks are dirty and sometimes dusty places to work. The conditions in this regard vary widely and are dependent upon how much effort the tank owner/operator is willing to expend in cleaning the floors in preparation for Magnetic Flux Leakage scanning and inspection. As an absolute minimum, a good water blast is necessary and all loose debris and scale should be removed from the inspection surface. The surface does not necessarily have to be dry but puddles of standing water need to be removed. The cleaner the floor, the better the inspection.
Storage Tank Surface Condition
Significant top surface corrosion and/or buckling of the tank floor plates represents a serious limitation to both the achievable coverage in the areas concerned and also the achievable sensitivity during the MFL inspection. While it is understood that very little can be done to improve this situation prior to inspection, it must be considered in the design of the MFL inspection equipment and its effect on the sensitivity of the inspection appreciated by both the owner/operator of the tank as well as the person conducting the examination. Any physical disturbance of the MFL scanning system as it traverses the tank floor will result in the generation of noise. The rougher the surface, the greater the noise and, therefore, a reduction in achievable sensitivity.
MFL Equipment Design Considerations
It is vital that Magnetic Flux Leakage NDT equipment used for storage tank floor inspection is designed to handle the environmental and practical field conditions that are consistently present. A piece of MFL inspection equipment designed in a laboratory and tested in ideal conditions will invariably have significant short comings in real world applications.
Powerful rare earth magnets are ideally suited for MFL inspections applications. They are more than capable of introducing the required flux levels into the material under test. Electromagnets by comparison are bulky and heavy. They do have an advantage in that the magnetic flux levels can be easily adjusted and “turned off” if necessary for cleaning purposes. Permanent magnet heights can be adjusted to alter flux levels, but the bridge requires regular cleaning to remove ferritic debris. The buildup of debris can have a significant impact on system sensitivity.
MFL inspection tools typically use one of two types of sensors: Coils and Hall Effect Sensors. They are both capable of detecting the magnetic flux leakage fields caused by corrosion on tank floors. There is a fundamental difference, however, in the way that they respond to leakage fields.
Coils are passive devices and follow Faraday’s Law in the presence of a magnetic field. As a coil is passed through a magnetic field, a voltage is generated in the coil and the level of this voltage is dependent on the number of turns in the coil and the rate of change of the flux leakage. From this, it is clear that speed will have some influence on the signals obtained from this type of sensor.
Hall Effect Sensors
Hall Effect Sensors are solid state devices which form part of an electrical circuit and, when passed through a magnetic field, the value of the voltage in the circuit varies dependent on the absolute value of the flux density. It is necessary to carry out some cross referencing and canceling with this type of sensor in order to separate true signals from other causes of large variations in voltage levels generated by the MFL inspection process.
There is disagreement within the industry as to which is the best type of sensor to use for this application. Hall Effect Sensors are undeniably more sensitive than coils. However, in this application, coils are more than adequately sensitive and are more stable and reliable. Hall Effect sensors prove to be too sensitive when surface conditions are less than perfect, which results in an unreliable inspection and the generation of significant false calls.
MFL Technique Application Considerations
It is virtually impossible to achieve 100% coverage using this technique due to the physical access limitations. The MFL inspection equipment should be designed so that it can scan as close as possible to the lap joint and shell. There are obviously compromises to be made as the wheel base of the scanner is an important consideration on tank floors that are not perfectly flat. Smaller scanning heads can be used in confined spaces to increase coverage.
Topside/Bottom Side Differentiation
Magnetic Flux Leakage cannot differentiate between the response from topside and bottom side indications. Some attempt has been made to use the eddy current signals from topside defects for the purposes of differentiation based on frequency discrimination. This is unreliable on real tank floors due to the uneven nature and lack of cleanliness of the inspection surface. In most cases, visual techniques are perfectly adequate for this purpose. Contrary to what is expected, the MFL response from a topside indication is significantly lower in amplitude than that from an equivalent bottom side indication. This means that, to some degree, the influence of the top side indications can be “tuned out” to allow a reliable assessment of the under floor condition.
Quantitative Assessment of Indications
Magnetic Flux Leakage is a qualitative, not quantitative inspection tool and is a reliable detector of corrosion on tank floors. Due to the environmental and physical restrictions encountered during real inspections, no reliable quantification of indications are possible. Amplitude alone is an unreliable indication of remaining wall thickness as it is more dependent on actual volume loss. Defects exhibiting various combinations of volume loss and through wall dimension can give the same amplitude signal. Couple to this the continually changing spatial relationship of magnets, sensor and inspection surface and it is absolutely clear that an accurate assessment of remaining wall thickness is virtually impossible. Truly quantitative results can only be obtained using a combination of Ultrasonic testing and Magnetic Flux Leakage.
The Single Level Threshold
Commercial expediency has brought about the implementation of accept/reject criteria using a single level threshold approach. MFE Enterprises, as a manufacturer of MFL inspection equipment, does not support this approach. As previously stated, the amplitude of signals alone is not a reliable indicator of remaining wall thickness. Significant indications can be completely missed especially in cases where the equipment does not incorporate some form of real time on line display. In order to carry out a reliable MFL inspection, the operator must have as much information as possible available to him in the form of an easy-to-interpret real time display. The use of a blind single threshold is absolutely indefensible in this application.
MFL Operator Training and Qualification Requirements
Currently, there is limited training available to users of the MFL equipment in regard to this application. MFE Enterprises Inc. recognizes this fact and offers initial basic training in magnetic flux leakage and the use of MFL inspection equipment on delivery of the scanner. This is obviously geared to our equipment and is quite specific. The ultrasonic prove up necessary must be carried out by personnel who are adequately trained and qualified. This is not just a “thickness measurement,” but rather a corrosion evaluation and the technician must have a full understanding of the technique that should be applied.
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